The present invention relates to methods of attaching a self-attaching element to a metal panel, particularly male and female fastener elements, such as studs, nuts, and the like, wherein the self-attaching element includes an annular or tubular self-riveting barrel portion which is deformed in a die member into a channel-shape which receives and retains the panel.
A new attachment system was recently developed primarily for automotive-type applications known as the "PIERCEFORM" fastening system. This attachment system is used for permanently attaching male and female fastening elements, such as bolts and nuts, to a metal panel in a press generating several tons of installation force. The self-attaching elements include an annular or tubular barrel which preferably is pierced through the panel and received in a die member which deforms the free end of the barrel radially outwardly into a channel-shape which receives and securely retains the pierced edge of the panel. The panel may, however, be pre-pierced or simultaneously pierced during the installation by a plunger received through the barrel of a female-type fastener. The attachment system may also be used to attach other elements to a metal panel, including ball studs, closures and the like.
The "PIERCEFORM" fastening system is particularly suitable for mass production applications, such as used by the automotive and appliance industries for interconnecting structural elements requiring high integrity of the joint formed between the self-attaching element and the metal panel. The self-attaching elements may also be simultaneously attached in a die press used for forming the panel or structural element.
In the most preferred applications, the free end of the barrel is deformed radially outwardly in a U or hook-shaped channel, and the pierced edge of the panel is simultaneously deformed into the developing U-shape, preferably substantially filling the channel and forming a secure mechanical interlock. In many applications of the "PIERCEFORM" bolt, for example, the strength of the joint is greater than the pull strength of the bolt. Examples of "PIERCEFORM" installation systems are disclosed in U.S. Pat. Nos. 4,459,073, 4,543,701, 4,555,838, 4,610,072, 4,633,560, 4,711,021 and 4,713,872, all of which are assigned to the assignee of the instant application.
At present, in a typical application of the "PIERCEFORM" system, the required installation force is between about 10 and 20 tons. As will be understood, orientation of the self-attaching element and die member, and very close tolerances, are very important to achieve a good installation. The installation force, which is composed of the forces required for piercing, forming of the barrel, and setting of the panel, results in very significant friction between the barrel and the self-attaching element, the die cavity and the panel. In a typical application of the "PIERCEFORM" stud bolt, for example, the pressure between the internal surface of the tubular barrel portion and the die surfaces may be over 50 tons per square inch. The frictional force generated by this pressure sometimes results in collapse of the barrel or stud, poor or incomplete installations and stress risers or faults which significantly reduce the integrity of the joint. Various attempts have been made to solve these problems, including orientation means, close tolerances, materials and coating of the die surfaces and various coatings on the self-attaching element, including petroleum-based lubricants, including kerosene and oil, phosphating the self-attaching element with a coating of oil, molybdenum disulfide coatings and white lithium grease. Coating of the die surfaces has not been successful because the friction between the barrel and the die surfaces caused by the high contact pressures removes the lubricant after a few installations. Further, the above-referenced lubricants have not been successful because of the very high contact pressures between the fastener barrel and the die element. Thus, there is a substantial need to improve the method of installing self-riveting fasteners of the type described above which avoids the problems of barrel collapse, incomplete installations and stress risers.